The present invention relates to a central shutter for cameras, especially professional cameras.
Such central shutters generally have a built-in objective consisting of a front element and a rear element and also have a polygonal diaphragm which is formed, for example, by five leaves, such leaves comprising an adjustable diaphragm aperture size system, and which can be actuated by a diaphragm actuator on which is formed a control curve cam, comprising a curved camportion with a linear characteristic main region and a steeply ascending end region, the latter being provided for pivoting the diaphragm leaves out of or peripherally outwardly beyond the clear passage of the shutter, and on which actuator there is also located, releasably and adjustably, an adjusting slide serving to limit the setting range of the diaphragm. The diaphragm actuator adjusting slide is also provided with a gripping handle and a scale indicator for the purpose of adapting the diaphragm setting mechanism to the specific f-number of the objective which is built into the particular shutter.
To make it possible to adapt the arrangement in the best possible way to given situations when photographs are taken with a professional or studio camera of the type mentioned above, professional photographers usually keep at their disposal several interchangeable objectives of different light intensity equipped with a central shutter, so that each forms a central shutter and diaphragm containing optical lens system objective unit interchangeably insertable in the particular camera being used. In this case, to make it simpler to attach the objectives or units to the camera and to remove them from it, they are each generally fastened on a plate, by means of which the particular objective and the shutter as a unit can be inserted releasably into a snap closure provided on the front carrier of the camera and hence can, as required, be readily exchanged for another such objective/shutter combination or unit.
Because they are equipped with a diaphragm mechanism, objectives or central shutters usable for this category of camera must, of course, also be provided with at least one concordant diaphragm scale which can be attached, as is customary, to the periphery and/or to the front plate of the shutter housing. As always, it is the responsibility or function of the shutter or objective manufacturer to attach the diaphragm scale appropriate for proper concordant use.
In view of the divergence of available shutters with different light passage sizes and objectives having lens systems with optical values differing from one another, this proper attachment of the diaphragm scale can be carried out, in practice, only after the purchase order documents or camera specifications have been submitted, which means that it is possible to equip a shutter with a given diaphragm scale only when a decision has first been made about the relative arrangement of the shutter and the objective to be particularly provided for use therewith. For this purpose, the shutter or objective manufacturer must keep an assortment of diaphragm scales available and from this assortment or inventory need only select the particular scale part suitable for the concordant use with the particular shutter/objective combination and in turn fasten it to the shutter housing.
To make it possible, moreover, to coordinate, without difficulty, the diaphragm mechanism with the diaphragm scale used, the diaphragm setting member of shutters of the above-mentioned design has been provided with an adjusting slide and this has been fastened variably or adjustably to the diaphragm actuator in its relative position in relation to the latter. Provided on the adjusting slide is a scale pointer which interacts with the diaphragm scale arrangement and which, during the diaphragm setting operation, with the aid of a plug gauge or test plug, has to be coordinated or precalibratingly adjusted with the predetermined light or exposure defining quantities or parameters of the shutter used, that is to say of its associated diaphragm and diaphragm scale.
When objectives of the design described above are equipped in a conventional way with a polygonal diaphragm, which is preferably formed by five leaves and which has been preferred over other designs of diaphragm arrangements because of its easy action and low production costs, the linear control curve characteristic of the control cam of the diaphrgm actuator must be selected so that when the actuator is actuated the free passage opening area or light passage area, and which forms in profile a pentagon with edges running in a straight line, corresponding to the geometrical form of the leaves, doubles in cross sectional light passage size from one diaphragm value to the next.
Furthermore, it is necessary to take precautions which, after the maximum size open diaphragm aperture still just lying within the fixed light passage of the objective has been reached, cause an absolute displacement of the diaphragm leaves out of or peripherally outwardly beyond the predetermined light passage of the objective when this condition is desired. As is known per se, this effect can be achieved in a simple way by providing in the end region of the diaphragm actuator cam control curve a steep ascent differing from the angle of ascent in the linear main region of the control curve.
However, the behavior is such that the effect originating from this steep ascent of the control curve can only be expected in the case of using a given objective, the free light passage of which corresponds to that of the central shutter or the associated control curve of the diaphragm mechanism. If objectives with a free light passage, the diameter of which is less than that of the central shutter, are utilized, the steep ascent provided in the end region of the linear control curve can no longer fulfill the function intended for it, specifically because as a result of the smaller diameter of the light passage of the objective, the end region of the control curve can no longer be activated or be functional to serve its intended purpose.